There has been a growing interest over the last decade in the nonlinear optical properties of conjugated organic and polymeric materials having delocalized .pi.-electron distributions. Nonlinear optical properties are the basis of newly emerging photonics technologies in which light works with, or even replaces, electrons in applications traditionally carried out by microelectronics. Organic materials are advantageous in photonics because organic molecular design and synthesis is flexible, and crystalline and thin-film phases are relatively easy to prepare.
Nonlinear optical responses can, in general, be classified as resonant or non-resonant, depending upon how close the optical frequencies employed are to the natural absorption frequencies of the material. Non-resonant, second-order processes, in which the response is proportional to the square of applied electric fields, arise only in materials that are noncentrosymmetric and posses both a suitably delocalized .pi.-electronic system and a large dipole moment. Second-order processes include second-harmonic generation, in which a material generates light at twice the frequency of the incident light, and the electrooptic effect, wherein an applied electric field changes the refractive index of the material and, therefore, alters the propagation properties of the incident light.
Compounds having nonlinear optical properties typically find use as single crystals or films layered upon a substrate, as main chains or side chains appended to a polymer core, or as "guests" dispersed in "host" polymers such as polyimides. The fabrication of semiconductors and other devices incorporating these structures can subject the nonlinear optical compound to temperatures on the order of -40.degree. C. to about 320.degree. C. These thermal requirements cannot be met by most known nonlinear optical compounds. The molecular units of crystals and films formed from such compounds typically are bound by relatively weak Van der Waals forces. Consequently, these crystals and films lack thermal and mechanical strength, and are highly susceptible to chemical attack and dissociation. For example, crystals of 2-methyl-4-nitroaniline, which are known to exhibit considerable nonlinearity, are so weakly bound that they actually sublime at room temperature.
Accordingly, there exists a need in the art for compounds that exhibit nonlinear optical properties yet can withstand semiconductor processing conditions. In particular, there is a need for compounds that are stable to high temperatures without detriment to their nonlinear optical coefficients.